DESIGN
OF A PHOTOCATALYTIC REACTOR FOR THE TREATMENT OF CYROMAZINE-CONTAMINATED WATER
USING HEMATOPORPHYRIN AS PHOTOSENSITIZER
DISEÑO DE UN
REACTOR FOTOCATALÍTICO PARA EL TRATAMIENTO DE AGUAS CONTAMINADAS CON CIROMAZINA
UTILIZANDO HEMATOPORFIRINA COMO FOTOSENSIBILIZADOR
Franklin Ramón
Vargas
[1] * ; Jessenia
María Ortega
[2] ; María
Alejandra Carrizales [3] ; Alexis
José Maldonado [4] & Beatriz
Celeste Angulo [5] ,
Recibido:
01 / 05 / 2023 – Recibido en forma revisada: 05 / 06 / 2023 -- Aceptado: 02 /
07 / 2023
*Autor
para la correspondencia.
Abstract: In this work, the
studies of the mechanisms of the photocatalytic oxidation of cyromazine using
hematoporphyrin as a photosensitizer agent were investigated. This oxidation
process was determined using UV-Vis spectrophotometry, GC-MS spectrometry, and
HPLC chromatography. The singlet oxygen generation was also determined.
Melamine and ammelide were identified as the photodegradation products of
cyromazine. A kinetic constant corresponding to a first-order equation was
obtained. A piston flow reactor was designed to purify water contaminated with
cyromazine, considering the need for a reactor that is easy to implement and
maintain, ideal for implementation in rural areas, and that ensure high
conversion per reactor volume.
keywords: Advanced
photooxidation, cyromazine, hematoporphyrin, photocatalysis singlet oxygen
Resumen: En este trabajo se
investigaron los estudios de los mecanismos de oxidación fotocatalítica de
ciromazina utilizando hematoporfirina como agente fotosensibilizador. Este
proceso de oxidación se determinó mediante espectrofotometría UV-Vis,
espectrometría GC-MS y cromatografía HPLC. También se determinó la generación
de oxígeno singlete. La melamina y la ammelida se identificaron como los
productos de fotodegradación de la ciromazina. Se obtuvo una constante cinética
correspondiente a una ecuación de primer orden. Se diseñó un reactor de flujo
pistón para purificar agua contaminada con ciromazina, considerando la
necesidad de un reactor fácil de implementar y mantener, ideal para ser
implementado en áreas rurales, y que asegure una alta conversión por volumen
del reactor:
Palabras clave: ciromazina,
fotocatálisis, fotooxidación avanzada, hematoporfirina, oxígeno singlete
1. Introduction.
Undoubtedly, pollution is the most
important cause of water scarcity. More than scarcity, water uselessness, since
it not only makes used water useless but in many cases it does so with the sources
where it is discharged. This is essential for the socioeconomic development of
man and for the healthy maintenance of ecosystems. As the population and its
development increase, it becomes necessary to expand water allocations for
domestic, agricultural, and industrial uses. The demand for this precious
liquid is increasing [1].
The conventional processes carried out in
most wastewater treatment plants are classified as primary, secondary, and
tertiary treatment. The primary or physicochemical treatment eliminates all
those substances that can be separated by deposition on the bottom of the
reactor (solids, colloids) or settling on the surface (oils or fats). Secondary
treatment is responsible for the removal of biodegradable organic matter through
microorganisms. Finally, tertiary treatment includes a series of techniques
aimed at separating from the water all those species that have not been
eliminated in previous treatments, such as anions and cations, and
non-biodegradable organic compounds.
Advanced oxidation processes (AOPs) are
highly efficient in water purification, being capable of generating transient
strongly oxidizing and non-selective species, mainly the hydroxyl, peridroxil
radicals, or singlet oxygen [2], to purify water under normal condition [3].
These techniques, developed over the last 20 years, represent one of the most
important alternatives to conventional wastewater treatment [4]. Photocatalysis
has been one of the most studied AOPs in recent years for the degradation of
organic pollutants in the aqueous phase. One of the main reasons for this
growing interest is that operating conditions are milder and more economically
viable. Another major benefit of using photocatalysts is the possibility of
using solar energy as a source of radiation [5].
The choice of the photosensitizer to use
depends on the main parameters of each treatment such as catalyst separation,
pH neutralization, reaction speed (the size of the plant depends on this) as
well as the cost of hydrogen peroxide [6]. Like other AOPs, photocatalysis is
an effective way of disinfecting water, of growing interest, which avoids the
use of oxidizing reagents and thus the formation of hazardous intermediates [7,
8].
In this work, the degrading action of
oxidizing species on cyromazine is investigated using hematoporphyrin (H2TCPP)
as a photosensitizing agent. A photosensitizer capable of generating singlet
oxygen under irradiation with light within the range of visible light. Kinetic
data from the given reaction were used for the design of a photocatalytic
reactor for the purification of cyromazine-contaminated water, at a low cost
and according to an environmentally friendly methodology. These techniques can
also be used in laboratory experiments to simulate the abiotic transformation
of contaminants in the euphotic zone (the zone with the highest light
reception), sometimes leading to potentially harmful transformation
intermediate products. Similarly, the role played by singlet oxygen in surface
waters is described. The latter, produced in aqueous solution by photo-excited
organic matter, has a micro-heterogeneous distribution in the solid phase and
in the hydrophobic nuclei of humic substances, and may have important
implications for the degradation of hydrophilic and hydrophobic contaminants.
Figure 1. Structures
of: cyromazine (left) and hematoporphyrin (right).
2. Experimental Methodology
All the irradiation processes were carried
out using a LuzChem LZC-4 illuminator (LuzChem Research Inc., Canada). The
irradiation time periods varied at 25 ºC, with emission in the UVA-Vis region,
320-600 nm. This was measured with a Digital Radiometer UVX (Thermo Fisher
Scientific, USA) after 1 h continuous illumination, (3.3 mW/cm2, 45,575 Lux/sec)
(radiation dose 4.5 J/cm2).
The analytical and HPLC-grade solvents
were purchased from Merck (Darmstadt, Germany). Hematoporphyrin, histidine,
rose bengal, and p-nitroso dimethyl aniline was purchased from Sigma-Aldrich
(St. Louis, MO, USA). Phosphate-buffered saline solution (PBS) pH 7.4 (0.01 M
phosphate buffer and 0.135 M NaCl). LiChrospher® RP-8e, (5 μm)
analytical column, at room temperature, was also used for the chromatographic
separation. This was purchased from Merck (Darmstadt, Germany). The analytes
were monitored at 240 nm. The elution was isocratic and the mobile phase
consisted of 0.1% TFA (CF3COOH) and methanol (CH3OH; 80:20 v/v) delivered
isocratically at a flow rate of 1mL/minute. Inlet pressure was between 180 and
190 bar. The injection volume was 20 μL. Melamine (99%) and Cyromazine
(99.8%) were purchased from Sigma-Aldrich. HPLC grade methanol Lichrosolv®
(MeOH, 99.8%) was purchased from Merck, ACN (99.99%) from Fisher Scientic and
trifluoracetic acid (TFA, CF3COOH, 99%) from ACROS Organics.
2.1. Cyromazine photodegradation.
The photodegradation of cyromazine was
followed by a UV-Vis spectrophotometry using a Perkin Elmer Lambda-35 UV-Vis
spectrophotometer (USA). The fluorescence spectra were registered with a Perkin
Elmer LS-45 spectrofluorophotometer. The structures of the isolated products
were elucidated by FT I.R. (Nicolet DX V 5.07) and mass spectra (Varian Saturn
2000) in connection with a Varian chromatograph equipped with a 30-m capillary
(CP-Sil, 8CB-MS). All irradiations (1.6 x 10-2 M) were also monitored by liquid
chromatography (HPLC, Waters Delta Prep 4000 and also with HPLC Perkin Elmer,
Serie 200) equipped with an analytic and a preparative C18- Bondapak and
analytic columns and UV-Vis detector) [8, 9].
The photodegradation of cyromazine was
determined using a spectrophotometer with UV-Visible detector, where products
that absorb at the same wavelength similar to the starting ones were observed.
For the identification and quantification of the different products obtained, a
high-performance liquid chromatograph (HPLC) equipped with a UV-vis detector
was used, this allowed the degradation to be followed and the species formed
together with the starting reagents to be observed. The identification of the
formed products was obtained by means of a mass spectrometer.
Photodegradation studies of cyromazine
.- Procedure
A 1.5 mL solution of each, cyromazine
(1x10-5 M) and H2TCPP (0.5x10-5 M) and was placed in a quartz cell with a 1 cm
optical pitch to measure its absorbance in the spectrophotometer. The
wavelength used was 207 nm corresponding to the maximum absorbance of
cyromazine. This procedure was repeated every 2 min for 40 min of irradiation
at 15,440 Lux. In addition, a standard was prepared that, like the other, the
absorbance was monitored every two minutes, but remained in the absence of radiation.
This pattern was performed to check the low toxicity of hematoporphyrin in the
dark.
To calculate the molar extinction
coefficient, mixtures of 1 mL of H2TCPP at 1x10-5 M and 1 mL of cyromazine at
different concentrations were prepared. Then, the absorbance was measured and
considering the Beer-Lambert law, the linear relationship between absorbance
and cyromazine concentration was determined. With this relationship, the
decrease in the concentration of cyromazine as a function of time was obtained.
The rate constant was determined from the slope of the line. It should be noted
that this procedure was performed five times to ensure the reproducibility of
the results.
2.2. Hematoporphyrin (H2TCPP)
photostability.
Solutions of 1.0 to 4.0 x 10-4 M H2TCPP
were subjected to irradiation, and the absorbance value was monitored every 15
minutes spectrophotometricaly. This experiment was carried out in order to
verify the thermal and photochemical stability of hematoporphyrin (H2TCPP) as
also to verify the intensity absorption of light in the region between 350 and
650 nm. Similarly, the UV-Visible spectrophotometer was also used to calculate
the reference quantum yield of singlet oxygen production [10].
The wavelength emitted by the lamps in a
Photoreactor Luzchem LZC-4 irradiation chamber equipped with 7 fluorescent
lamps LZC-420 (centered approx. at 420 nm) and 7 fluorescent lamps LZC-355 (UVA
lamps centered approx. 355 nm), are where hematoporphyrin effectively has a maximum
absorbance (396 nm).
2.3. Determination of fluorescence
quantum yield (ΦF).
H2TCPP emission and energy transfer
studies were carried out under argon on saturated dichloromethane solutions of
this porphyrin using a Perkin Elmer LS-45 spectrofluorophotometer. Relative
fluorescence quantum yields at room temperature were determined by comparing
the corrected fluorescence intensity of H2TCPP with that of
meso-tetraphenylporphyrin in chloroform ΦF = 0.110, rhodamine B (1.0 x 10-6
M, ΦF =
0.69 / ethanol), and with that of quinine bisulfate in 0.05 M H2SO4 (ΦF =
0.55) or pyrene (ΦF =
0.38 / CH2Cl2) [10].
The fluorescence spectrum consists of a
peak at 625 nm and a second, less intense band at 675 nm. The 1O2 quantum
yields were found to be 0.61 ± 0.03 for hematoporphyrin.
2.4. Determination of singlet
oxygen generation.
The histidine test was performed in order
to determine the quantum yield of singlet oxygen production of hematoporphyrin
using rose bengal as a reference, whose theoretical quantum yield value is
reported [11].
The experimental methodology used for the
Histidine Test consisted of placing 1mL of hematoporphyrin, 1mL of histidine,
and 0.5 mL of nitrosoaniline (all three at a concentration of 0.1 nM) in a
quartz cell. The mixture was subjected to irradiation (LuzChem equipment) and
the advance in singlet oxygen production was followed by the decrease in the
characteristic band of nitrosoaniline measured at 440 nm in the
spectrophotometer. The quantum yield of singlet oxygen production of
hematoporphyrin was found using the quantum yield of rose bengal as a
reference. Determination of the production of 1O2 of the H2TCPP was taken as a
function of the absorbance variation in the time of irradiation at 15440 Lux.
The quantum yield of 1O2 production of H2TCPP was determined to be 0.65 for an
irradiation time of 110 minutes. This value shows that this porphyrinic
compound generates enough singlet oxygen to be used as a photosensitizer.
The determination of photoinduced singlet
oxygen 1O2 generation and its quantum yield was also carried out for
hematoporphyrin with 9,10-diphenylanthracene. This compound is an efficient 1O2
scavenger and its disappearance kinetics can be followed by UVA-Vis
spectrophotometry by its gradually decreasing absorbance at 374 nm [12, 13].
Irradiation of hematoporphyrin with visible light under oxygen was carried out
in the presence of diphenylanthracene in chloroform. Irradiation at these wavelengths
does not affect diphenylanthracene. The 1O2 quantum yields were found to be
0.61 ± 0.03 for hematoporphyrin.
3. Results and Discussion
3.1. Hematoporphyrin (H2TCPP)
photostability.
In order to know the wavelength that
corresponds to the maximum absorbance of cyromazine, 207.5 nm, an absorption
spectrum was performed from 200 to 650 nm.
To verify the use of hematoporphyrin as a
photosensitizer, an absorption spectrum was taken, clearly observing the
intense band located between 350 and 700 nm (See figure 2). The intensity of
the band indicates that UV-Visible radiation can be used to excite this
photosensitizer. Figure 2 shows that the absorbance of hematoporphyrin is not
affected by irradiation. This means that this photosensitizer molecule is not
broken down by light. The thermal stability is verified since the sample, when
subjected to irradiation, increases its temperature from 26 °C to 53 °C and the
absorbance is maintained. Finally, it should be clarified that H2TCPP, in
addition to being able to be used as a photosensitizer, is commercial and
non-toxic, so its use favors both the economic and environmental aspects.
Figure 2. Absorption
spectra and thermal and photochemical stability of H2TCPP.
3.2. Using a high performance
liquid chromatograph (HPLC).
Samples of the H2TCPP mixture (1x10-5 M)
were taken at various concentrations of cyromazine (from 0.5x10-5 M to 9x10-5
M). They were injected into the chromatograph manually and an area was obtained
for each concentration, with them a linear adjustment of the area as a function
of concentration was performed to obtain the equation of the straight line that
corresponds to the calibration curve. Then, aliquots of the mixture of H2TCPP
and cyromazine were taken at different irradiation times at 15440 Lux from (0
min to 100 min with 10 min intervals) to obtain the area for each irradiation
time. Subsequently, using the calibration curve obtained previously, the
concentration at the different irradiation times was determined. Finally, the
logarithm of the ratio of the concentration over time between the initial as a
function of time was plotted.
Applying regression through least squares
it was possible to determine the first-order kinetic constant. The following
figures show the absorption spectrum of cyromazine and its HPLC chromatogram.
The cyromazine solution was irradiated for 70 min at 1544 Lux.
Figure 3. (a)
HPLC chromatography of cyromazine before irradiation. (b) HPLC chromatography
of cyromazine after irradiation.
3.3. Photodegradation of cyromazine
en presence of H2TCPP.
It is noteworthy that under Argon
atmosphere the cyromazine photodegradation reaction does not occur completely.
Only the Melamine compound was determined as a product of the photosensitized
reaction (Figure 4). Which could indicate a photoinduced electron transfer
reaction from hematoporphyrin to cyromazine. In the presence of oxygen, the
formation and subsequent intervention of singlet oxygen are evident. The
formation of two photoproducts were determined by GC-mass: melamine and
ammelide.
Figure 4. Photocatalytic
reaction products (left) melamine, (right) ammelide.
Once the kinetics of the reaction was
obtained, a mass spectrum was taken of the solution obtained after 70 minutes
of irradiation in order to know the molecular weight of the compounds formed
and to be able to identify them.
This study was carried out through a mass
spectrometer where the relative abundance of the compound is reported based on
the charge/mass ratio (m/z), obtaining: the main product would be given by the
peak of 125.17, which is equivalent to a molecular weight of 126.17 g/mol which
coincides with melamine and the other representative peak is located at 127.16
which is equivalent to a molecular weight of 128 g/mol corresponding to
ammelide) (8, 9). The major photoproduct would be given by the peaks of 125.17
m/z (100, M+), 109.16 (46, M+ - NH2), which are equivalent to a molecular
weight of 126.17 g/mol which coincides with melamine. The other representative
peaks are located at 127.16 m/z (60, M+), 110 (18, M+-OH) which are equivalent
to a molecular weight of 128 g/mol corresponding to the ammelida (Figure 4). In
a lower proportion, the formation of hydroxylamino derivative of melamine was
also detected, 142 m/z (5).
3.4. Determination of reaction
kinetics.
In the graph shown, it is clearly observed
that the concentration decreases exponentially with respect to time, so based
on the following equation, it is concluded that it is first-order kinetics.
Theoretically, this result is reasonable, since it is a reaction of the form:
C + B ------- Products
Where C is cyromazine and B is the oxygen
present in the medium. As oxygen is considered a reactive in excess, its
concentration variation will be negligible compared to that of cyromazine,
therefore, the kinetics will depend only on the concentration of cyromazine.
After performing a least squares fit the
results obtained and considering that the points fit the behavior given by the
given equation, the kinetic constant corresponding to a first-order equation
was obtained. Being k = 0.0453 ± 0.0005 with units of s-1 because it is a
first-order kinetics.
d [Cyromazine] / dt -------
0.0453 [Cyromazine]
Figure 5. Photocatalytic
degradation of cyromazine (1x10-3 M) in the presence of hematoporphyrin (1x10-5
M) (left) and linear regression of the kinetic equation (right).
3.5. Photocatalytic reactor design.
For the design of the photocatalytic
reactor, a continuous flow reactor is considered in the first place based on
the system under study, specifically, piston type (FPR, flow photocatalytic
reactor), which ensures a greater contact area with the energy source, easy
implementation, and maintenance, in addition, allows a high conversion by the
differential in the volume of the reactor. For its design, it is considered
that the pressure and temperature are constant throughout it. Following the
results obtained from the cyromazine degradation study, the kinetics of the
reaction were determined and the reactor was designed, making a balance of
matter in a differential volume of the reactor, to finally obtain an expression
for the volume of the reactor as a function of the flow of water to be
purified.
The following figure 6 schematizes the
algorithm for reactor design as a flow chart.
Figure 6. Algorithm
for the design of the photocatalytic reactor.
Once the values of the kinetic constant
and of the conversion are known, the equation deduced to calculate the volume
of the reactor is based on the initial flow rate at its inlet. This variable
can be manipulated depending on the amount of water to be treated. In addition,
the volume is directly proportional to the treated flow rate, so its choice
also depends on the volume required for the reactor. In addition, the volume is
directly proportional to the treated flow rate, so its choice also depends on the
volume required for the reactor (see figures 7 and 8).
Figure 7. The
volume of the tubular reactor is a function of the flow rate of treated water.
Figure 8. Schematic
diagram of the reactor.
4. Conclusions
Water purification by UV is a safe,
versatile, and competitive method compared to traditional purification methods.
This photosensitized and porphyrin-catalyzed oxidation technique in aerobic environments
has also been shown to have (current research on processes) highly effective
bacterial inactivation. Its compatibility with other purification techniques
enables many applications for the treatment of well water, wastewater and
irrigation systems. The use of this technology and the development of applied
research in water purification processes provide a very fertile field for many
future projects. The importance of photochemistry in the degradation of
pesticides under ambient conditions has been widely demonstrated, emerging as a
possible technological solution for the complete mineralization of pesticides,
with higher efficiencies and lower costs than conventional methodologies.
However, it would be convenient to mention that the experimental studies to
date have been directed towards the conditions related to the chemical
characteristics of water. A better understanding of the natural system could
provide parameters that optimize the current methods of degradation of
pesticides, among other chromophore contaminants.
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